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Difference between Moving Iron and Moving Coil Instrument
There are many quantities in an electrical and electronic circuit that need to be measured. For this purpose, many types of measuring instruments have been developed such as moving iron instruments, moving coil instruments, dynamometer type instruments, etc.
In this article, we will learn about the key differences between moving iron and moving coil instruments by considering various parameters such as basic definition, symbol, uses, scale, working principle, power consumption, etc. But, before discussing about the differences, it is advantageous to know about these instruments in short.
What is a Moving Coil Instrument?
A type of measuring instrument in which a coil moves in a magnetic field created by a permanent magnet to perform its operation is known as moving coil instrument or M. C. instrument. Since in a moving coil instrument, the permanent magnets produce the operating magnetic field, thus it is also called permanent magnet moving coil instrument or PMMC instrument.
Moving coil instruments operate on the principle that when a coil that is free move is placed in a magnetic field of a permanent magnet, it experiences a force and rotates. These types of instruments can only measure the direct current (DC). These do not work with the alternating current (AC).
What is a Moving Iron Instrument?
The types of measuring instruments that use a core made of soft iron as moving part to perform the measurement of current or voltage is known as moving iron instrument or MI instrument.
The working principle of moving iron instruments is based on the fact that a magnet attracts the iron. In case of moving iron instruments, the electromagnets produce the operating magnetic field. The soft iron core placed in this magnetic field experiences a force of attraction and the strength of the force depends on the strength of the magnetic field. The moving iron instruments can measure both alternating current and direct current.
Difference between Moving Coil Instrument and Moving Iron Instrument
Both moving coil and moving iron instruments are widely used in electrical and electronics measurements. The following table compares and contrasts the various features of these two instruments −
| Basis of Difference | Moving Coil Instrument | Moving Iron Instrument |
|---|---|---|
| Definition | A measuring instrument which involves the movement of a coil in a magnetic field of a permanent magnet to measure the electric current or voltage is called moving coil instrument or M. C. instrument or PMMC instrument. | The measuring instrument in which a core of soft iron moves in a magnetic field of an electromagnet to measure the electrical current or voltage is called moving iron instrument or M. I. instrument. |
| Operating principle | The operation of the moving coil instrument is based on the fact that a current carrying coil experiences a force that tends to move it when placed in a magnetic field of a permanent magnet. | The operation of a moving iron instrument is based on the magnetism, i.e. magnetic field attracts a magnetic material such as iron, etc. |
| Circuit Symbol |
|
|
| Reading scale | The moving coil instrument has a uniform reading scale. | The moving coil instrument has a non-uniform scale, which is cramped at starting end. |
| Measurement | Moving coil instruments can measure direct current (DC) only. | Moving iron instruments can measure direct current (DC) as well as alternating current (AC). |
| Deflection of pointer | In the moving coil instruments, the deflection of pointer is directly proportional to the current in the coil, i.e. θ ∝ I. | In the moving iron instruments, the deflection of the pointer is directly proportional to square of current, i.e. θ ∝ I2. |
| Accuracy | Moving coil instruments are comparatively more accurate. | Moving iron instruments are less accurate than M. C. instruments. |
| Construction | The construction of a moving coil instrument is relatively complex because it uses a moving coil and stationary magnetic field. | The moving iron instrument has simple construction because it uses a stationary coil. |
| Robustness | Moving coil instruments are very sensitive in construction, i.e. these are less robust. | Moving iron instruments are robust in construction. |
| Damping | Eddy current damping is provided in moving coil instruments. | Air friction damping is provided in moving iron instruments. |
| Controlling torque | Moving coil instruments use a control spring to provide the controlling torque. | In moving iron instruments, the controlling torque is provided by either gravity control or spring control. |
| Magnet | Moving coil instrument uses permanent magnet. | Moving iron instrument uses electromagnet. |
| Sensitivity | Moving coil instruments are more sensitive. | The sensitivity of moving iron instrument is less. |
| Coil construction | The coil of a moving coil instrument is always made of thin wire with less number of turns. | The construction of coil for a moving iron instrument depends on the magnitude of current or voltage measured. |
| Rotating element | In a moving coil instrument, the rotating element is a coil of fine wire. | Moving iron instrument has a core of soft iron as a rotating element. |
| Basic range of current | The basic current range for a moving coil instrument is 10 μA to 100 mA. | The basic range of current for a moving iron instrument is relatively high, from 10 mA to 100 A. |
| Power consumption | Moving coil instruments consumes less power. | Moving iron instruments consumes high power. |
| Used as | Moving coil instruments are mainly used as a measuring instrument for DC. | Moving iron instruments are mainly used as an indicating instrument. |
| Hysteresis loss | The hysteresis loss does not occur in a moving coil instrument. | The hysteresis loss takes place in a moving iron instrument. |
| Cost | Moving coil instruments are expensive. | Moving iron instruments are relatively less expensive. |
Conclusion
The most significant difference between moving iron and moving coil instruments is that a moving iron instrument can measure both AC and DC, whereas a moving coil instrument can measure DC only.
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